Looks like the bug is easily reproducible when the device gets connected before it's registered to the network, but I was able to reproduce the problem after connecting it manually after it's registered as well, but in this case, not always reproducible.
One thing I noticed (not sure if related) is that sometimes I get the ringtone as a2dp, sometimes I don't get nothing, and the device supports hfp, but it doesn't work on bluez5 as it seems. It does work on bluez4.
Looks like the bug is easily reproducible when the device gets connected before it's registered to the network, but I was able to reproduce the problem after connecting it manually after it's registered as well, but in this case, not always reproducible.
One thing I noticed (not sure if related) is that sometimes I get the ringtone as a2dp, sometimes I don't get nothing, and the device supports hfp, but it doesn't work on bluez5 as it seems. It does work on bluez4.